1. Field of the Invention
The present invention relates to a both-side document reading apparatus used, for example, in a scanner apparatus, copier, printer, facsimile or the like, and a both-side document reading method to be implemented in the apparatus. More particularly, the invention relates to a countermeasure for properly obtaining read data of density.
2. Description of the Related Art
Conventionally, there is known both-side document reading apparatus having a function to read images and characters on the both surfaces of a document by the reading means arranged on the both sides of a document transport path disclosed in Japanese Unexamined Patent Publications JP-A 9-321947 (1997) and JP-A 10-98588 (1998), for example.
Specifically, JP-A 9-321947 discloses a technical idea having reading means having light sources arranged on the main and back sides of a reading document to illuminate the surfaces of the document with light, so that the light source on the main-surface side and the light source on the back-surface side are alternately switch-controlled on a scanning-line basis and the reading at the document main and back surfaces is alternately switched interactively with putting on and off of the light sources, thereby avoiding optical interference. Namely, back reflection is avoided from occurring on the document read data of the other reading means due to the light source of one reading means.
On the other hand, JP-A 10-98588 discloses a technical idea that the reading means are deviated in positions not opposed to each other in order to prevent interference between the illumination from a light source of the reading means for reading a document main surface and the illumination from a light source of the reading means for reading a document back surface, thereby avoiding optical interference.
In the meanwhile, the technology described in JP-A 9-321947 must have an extremely high responsibility in putting on and off the light sources because both the light sources are controlled in putting on and off in order to put one only of both the light sources on a scanning-line basis. As a result, there has been the restriction of incapability of using a low responsible light source. Also, where increasing the document transport rate in order to increase the number of reading documents per unit time, there is a need for light sources having further higher responsibility thus incurring cost mounting-up. Furthermore, because there is a limitation in light-source responsibility, the document reading rate suffers the restriction to light-source responsibility thus imposing a limitation in increasing the document reading rate.
The technology of JP-A 10-98588 displaces in position the document reading means arranged on the both sides of the document transport path in a document transport direction thereby making possible to avoid the problem in the JP-A 9-321947. In the meanwhile, in the technology described in JP-A 10-98588, when the reading at document one surface is completed, the light source of the document reading means having read precedent one surface is put off so that, after completing the reading at the other surface with corresponding delay to the displacement, the light source of the document reading means having read at the other surface is put off. In the case that both of the document reading means are closely provided in order to reduce the apparatus size (where decreasing the displacement), there is a possibility that the light of the light source of the one document reading means be incident on the other document reading means. In this case, despite the reading at the other surface is not completed, the light source on the side reading has precedently completed is put off. Thus, the data of density in reading at the other suffers the effect of the change in light amount. Namely, read data of density changes at the point of light-amount displacement as a boundary, causing a problem of incapability of obtaining correct document reading data.